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Thread: help needed to choose 2 dioodes.

  1. #1

    help needed to choose 2 dioodes.

    I have the following circuit block:

    Click image for larger version. 

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ID:	26751

    I am building a prototyping board that allows me allot of options. I may be skipping this part of the circuit and driving AAT-REQ-5V or ATT-REQ-3.3V directly. Should I protect these transistors etc by placing diodes? My memory from back in the day is failing me. What value/type of diode would do this for me?

    The unmarked trace on the left is a 5v logic line ATTREQ from an STE Eurocard bus.

  2. #2
    Senior Member
    Join Date
    Jul 2020
    Q2 has nothing to limit the base current, so it'll be immediate toast if the ATTREQ line is driven to 5V. Perhaps 10k
    base resistor will fix that.

    The ATTNREQ line has a series resistor, and is open-collector anyway so that's very robust already.
    Not sure the 47pF speed-up cap will do much as the rise-time at Q1's collector will be limited by the 10k
    pullup mainly. That could be lowered to more like 1k for snappier response if this matters.

    With series resistors I see no need for diodes unless these lines might get driven negative, which seems
    unlikely - but you can always add Schottky clamps to the rails for any logic signal going on/off board if
    you know the relevant rail voltage for each:

    Its the done thing and a good habit to get into.

  3. #3
    @MarkT Thanks for the advice, how does this look?

    Attachment 26757

    I was thinking if two diodes to just keep the line from being driven in reverse as I have patch points everywhere to modify the protoboard im building. D3 and D4:

    Attachment 26758

    In any event, eralier tody I found some documention about the STE bus and I now realise I have a wrong conclusion about the nature of the ATTREQ lines, in the case of the board I am "replacing" it actually boradcasting "to the bus" so these shematics are totally wrong anyway, but none the less its good to learn how to do this anyway.

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